Hierarchical star formation across the grand-design spiral NGC 1566

Abstract

We investigate how star formation is spatially organized in the grand-design spiral NGC 1566 from deep HST photometry with the Legacy ExtraGalactic UV Survey (LEGUS). Our contour-based clustering analysis reveals 890 distinct stellar conglomerations at various levels of significance. These star-forming complexes are organized in a hierarchical fashion with the larger congregations consisting of smaller structures, which themselves fragment into even smaller and more compact stellar groupings. Their size distribution, covering a wide range in length-scales, shows a power-law as expected from scale-free processes. We explain this shape with a simple "fragmentation and enrichment" model. The hierarchical morphology of the complexes is confirmed by their mass-size relation which can be represented by a powerlaw with a fractional exponent, analogous to that determined for fractal molecular clouds. The surface stellar density distribution of the complexes shows a log-normal shape similar to that for supersonic non-gravitating turbulent gas. Between 50 and 65 per cent of the recentlyformed stars, as well as about 90 per cent of the young star clusters, are found inside the stellar complexes, located along the spiral arms. We find an age-difference between young stars inside the complexes and those in their direct vicinity in the arms of at least 10 Myr. This timescale may relate to the minimum time for stellar evaporation, although we cannot exclude the in situ formation of stars. As expected, star formation preferentially occurs in spiral arms. Our findings reveal turbulent-driven hierarchical star formation along the arms of a grand-design galaxy.

GASP XIII. Star formation in gas outside galaxies

“…2) Timescale arguments rule out the possibility that the photons ionizing the gas in the tails originate from massive stars in the disks of these galaxies. Based on this, as well as on our molecular gas detections in the tails (Moretti et al 2018b) and our deep FUV and NUV imaging (George et al 2018), we conclude that star formation occurs in situ in the tails, within the Hα clumps we identify.…”

“…In the same fashion, the FUV and NUV light of the O and B young stars in the clumps of the tails was directly observed with UVIT@ASTROSAT in JO201, one of the galaxies in our sample (George et al 2018), where we found a remarkable agreeement between the SFR of individual clumps derived from the FUV and from Hα. GALEX data of several GASP jellyfishes, albeit at much lower spatial resolution, support the same scenario.…”

“…Turbulence appears to play an important role (e.g. Elmegreen et al 2014;Gouliermis et al 2017) and ambient conditions influence the formation of molecular clouds and their properties as a consequence of radiative, thermal, magneto-hydrodynamical and dynamical processes.…”

The spatial relation between young star clusters and molecular clouds in M51 with LEGUS

“…M 51 (NGC 5194 and NGC 5195) is part of the Legacy Ex-traGalactic UV Survey 1 (LEGUS, HST GO-13364; Calzetti et al 2015a), a Cycle 21 HST treasury program of 50 nearby (∼3.5-16 Mpc) galaxies in five UV and optical bands (NUV, U, B, V, and I) with the goal of investigating the connection between local star forming regions -as traced with young stellar clustersand global star formation within the nearby universe. Results with the LEGUS datasets so far include support for a hierarchical star formation process (Elmegreen et al 2014;Gouliermis et al 2017;Grasha et al 2015Grasha et al , 2017a with the age and size distribution of the hierarchies driven by turbulence (Gouliermis et al 2015b;Grasha et al 2017b); investigation of the effect on the evolution of galaxies from the radiative and mechanical feedback of star clusters (Calzetti et al 2015b); tests for variations in the initial mass function of star clusters (Krumholz et al 2015;Ashworth et al 2017); test spiral density wave theory (Shabani et al 2018); and extinction maps using stellar catalogues to investigate variations in the dust-to-gas ratio with metallicity (Kahre et al 2018). In this work, we perform a cross-correlation analysis between the star clusters from the LE-GUS catalogue and the GMC catalogue from the PAWS project in M 51.…”

“…In such a distribution, individual components of a population are more likely to appear near other members than if they were randomly distributed. Observations have directly measured the correlation in the spatial distribution between young stars, stellar clusters, and associations (e.g., Gomez et al 1993;Zhang et al 2001;Oey et al 2004;de la Fuente Marcos & de la Fuente Marcos 2009;Karampelas et al 2009;Bianchi et al 2012;Gouliermis et al 2015bGouliermis et al , 2017Gouliermis 2018;Grasha et al 2015Grasha et al , 2017aSun et al 2017a,b;Rodríguez et al 2018), providing overwhelming evidence for the coherence of star formation over multiple scales and across most galactic environments.…”